Clean up grammar

[davepagurek]

• Make rule names longer
• Add comments to rules in the grammar
• See if it's possible to make rules take up more than one line in the grammar, maybe by doing our own preprocessing
• Make to_html be const
• Use const Json
• Make a readme documenting the things we've learned for using peglib (e.g. named tokens can't be used in our case, ~ ignores the token)